Parathyroid secretory protein (SP-I) is an acidic glycoprotein that is co-stored and co-secreted with parathyroid hormone (PTH) under calcium control. It has been shown to be chemically similar, if not identical to Chromogranin A, the major protein of the adrenal medulla chromaffin granule, that is co-stored and secreted with epinephrine and other granule contents. Moreover, SP-I has been found in every endocrine cell examined-- but never in an exocrine or epithelial cell. We and others speculate that SP- I (or chromogranin A) plays an important but yet-to-be-defined role in the processing, packaging or secretion of endocrine proteins, possibly by providing an intracellular "traffic signal". We propose two approaches to test this hypothesis. In the first approach we will construct recombinant genomes containing cDNA encoding either SP-I or PTH and insert them into the AR4- 2J exocrine pancreas cell and into the MDCK polarized epithelial kidney cell, neither line of which expresses SP-"I. We will then test the effect of the introduction of the SP-I cDNA on processing and secretion of endogenous protein(s) and on that of the exogenous pth. We will also evaluate posttranslational modifications to the expressed SP-I. In the second approach, we will attempt to block the synthesis of SP-I in an endocrine cell by introducing antisense cDNA to mRNA for SP-I. We will construct the suitable genome containing antisense SP-I and transfect the TT cell, a medullary thryoid carcinoma-derived line that contains expresses both SP-I and calcitonin. We will then evaluate the effect of this procedure on the processing and secretion of calcitonin. These approaches will provide alternative means to determine if and how SP-I affects the secretion of exported proteins.